While the invention is subject to a wide range of applications, it particularly relates to metal cord, such as that used to reinforce elastomers. More particularly, the present invention is directed to a single strand cord of compact construction used to reinforce tires.
It is known to manufacture single strand, metal cord in a single operation wherein conventional cord is made of filaments having the same diameter twisted together in the same direction and having the same lay length. The cord is said to have a compact cross section which is generally the same over the length of the cord. The filaments in the cross section are arranged in concentric layers in which the filaments are tangential to all the filaments surrounding an individual filament. The single operation produces a cord having a single, compact strand. The single strand, compact cord has the advantages of a reduced cord diameter. These advantages include: (1) a calendared ply having a reduced thickness and therefore requiring less calender rubber; (2) a potential increase in the ends per inch for a given width of ply; and (3) an increased ply strength resulting from an increase in the ends per inch. The uniform cross section is thought to more uniformly distribute the load carried by the cord to each individual filament. The result is a higher breaking load. Further, the cord is thought to have improved fatigue resistance and greater flexibility.
In a conventional pneumatic radial tire using steel cords, as described above for reinforcement, the fatigue properties of the carcass ply and belt layer are degraded mainly by material fatigue due to repeated strain and fretting wear in the contact portion between adjacent filaments. Both of these problems, as discussed in U.S. Pat. Nos. 4,707,975 ('915) and 4,788,815 ('815), are said to be alleviated by penetration of rubber into the inside of the cord. Then, the rubber layer interposed between the steel filaments prevents rubbing between adjacent filaments, the so called "fretting wear".
The penetration of the rubber is difficult to achieve in multi-layer, structured cords. When some of the filaments are not covered with rubber due to incomplete rubber penetration, the fatigue properties of the cord are not substantially improved. The '915 and '815 patents attempt to overcome this problem by forming a central base structure composed of 1 to 4 steel filaments with a larger diameter than the diameter of each of the plurality of steel filaments forming at least one coaxial layer arranged around the central base structure so as to adjoin them to each other. All of the steel filaments are twisted in the same direction at the same pitch.
A modification of the above described conventional cord is set forth in U.S. Pat. No. 4,608,817 ('817), assigned to The Goodyear Tire and Rubber Co, a common assignee with the present invention. In the '817 patent, a structural relationship exits between the core and the surrounding layers of the cord. That is, a single strand of metal cord is produced from filaments of a similar diameter twisted together in the same direction and having the same lay length. The cord is constructed of a core of two or more filaments; at least one layer of filament placed on and about the core; and at least one filament in the core being positioned so that the filaments in first layer are next to the core. Further, at least one filament in the first layer may be positioned with the filaments in the second layer, or each additional layer of filaments may have at least one filament from the adjacent inner layer positioned with the filaments in each additional layer. The cord constructions can be expressed generally by N+(N+X)+(N+X+Y), where N is the number of filaments in the cord core and is greater than 1, (N+X) is the number of filaments laid on the core filaments where 1&lt;X&lt;6, and (N+X+Y) is the number of filaments on the (N+X) number of filaments where 0&lt;Y&lt;7. Additional filaments can be laid on as above in accordance with the series N+Y+X+Y, etc. and a fewer number of filaments can be expressed simply by N+(N+X) within the ranges given above for X and Y. An example of a cord using the above expression would be a 27X where N=5, X=3, and Y=6 to give 5+(5+3)+(5+3+6) or 5+8+14.